Polypropylene sheet is formed by drawing an extruded curtain melt through opposed dies that are finely adjustable to determine sheet characteristics. The surface patterning of the resulting sheet is determined by a pass over a large stainless steel roll having an appropriate complementary surface finish. The rolls are expensive in the sense that, although replacement for wear is only occasional, they are easily damaged during roll handling or machine adjustment and when damaged, even in a minor way, are inevitably written off.
Polypropylene sheet produced in this way has found a wide variety of applications and a large proportion of these involve printing of the sheet. To optimise offset printing, for example, a high quality finish is desirable which is sufficiently matt to retain the ink and yet has a surface topography that achieves optimal uniformity of ink spread. Magnification of printed surfaces of this kind will often reveal gaps in the ink coverage which arise from interaction between the ink liquid, which has a high surface tension, and fine topographical features of the surface. Such ink gaps may not be readily apparent to the naked eye but nevertheless adversely affect print quality.
A further consideration is that polypropylene accurately replicates surfaces it contacts and thus any imperfections in the finishing roll surface will be faithfully reproduced in the surface of the plastic sheet.
A known method for finishing the surface of stainless steel rolls is by grit blasting with alumina particles at a blasting nozzle air pressure of 60 psi. A first series of passes using alumina grit of a larger size range is followed by a series with grit of a lower size range and then a single pass of the same larger size range. These are all carried out at a uniform blasting nozzle air pressure. The process is completed with a single pass with fine glass beads, of size an order of magnitude lower than the alumina and at an air pressure lower than for the alumina passes.
It is an object of the invention to provide a process for finishing a metal surface in order to achieve optimum uniformity of the surface with finite but minimal height variations.